Process for the manufacture of ferroelectric materials

Abstract

A process for the preparation of a niobium compound of formula (I): D α Nb β E γ O 3-δ   (I) wherein D is an alkali metal (e.g. Li, Na, K, Rb, Cs and/or Fr), alkaline earth metal (such as Ba, Ca, Mg and/or Sr), La and/or Bi and may be present as a mixture of two or more metals; E is Ta, Sb and/or Fe and may be present as a mixture of two or more metals; α is a positive number β is a positive number γ is zero or a positive number δ is a number 0≦δ≦0.5; and wherein the formula (I) has the perovskite or tungsten bronze structure; comprising spray pyrolising a solution, for example an aqueous solution, comprising metal (D) ions, Nb ions and if present, metal (E) ions.

The invention claimed is: 
     
       1. A process for the preparation of an alkali metal niobium oxide of formula (Ia):
   A 1-2x-3w B x C w Nb 1-y-3/5z-u Ta y Fe z Sb u O 3   (Ia)
 
 wherein 
 A is Li, Na, K, Rb, Cs, Fr, or a mixture thereof; 
 B is Ba, Ca, Mg, Sr, or a mixture thereof, 
 C is La, and/or Bi; 
 0≦u≦0.3; 0≦w≦0.3; 0≦x≦0.3, 0≦y<1 and 0≦z≦0.3, the values of u, w, x, y and z being independent of each other with the proviso that the sum of y+3/5z+u is less than 1 and with the proviso that the sum of 3w+2x is less than 1; 
 comprising spray pyrolising an aqueous solution, comprising alkali metal (A) ions, Nb ions and optionally alkaline earth ions (B), Ta Sb, La, Bi, and/or Fe ions, to form a powder and calcining said powder. 
 
     
     
       2. A process as claimed in  claim 1  for the preparation of an alkali metal niobium oxide of formula (Ib):
   A 1-2x-3w B x C w Nb 1-y-3/5z Ta y Fe z O 3   (Ib)
 
 wherein 
 A is Li, Na, K, Rb, Cs, Fr, or a mixture thereof; 
 B is Ba, Ca, Mg, Sr, or a mixture thereof, 
 C is La, and/or Bi; 
 0≦w≦0.3; 0≦x≦0.3, 0≦y<1 and 0≦z≦0.3, the values of w, x, y and z being independent of each other with the proviso that the sum of y+3/5z is less than 1 and with the proviso that the sum of 3w+2x is less than 1; 
 comprising spray pyrolising a solution, for example an aqueous solution, comprising alkali metal (A) ions, Nb ions and optionally alkaline earth ions (B), Ta, La, Bi, and/or Fe ions to form a powder and calcining said powder. 
 
     
     
       3. A process as claimed in  claim 2  for the preparation of an alkali metal niobium oxide of formula (Ic):
   A 1-2x B x Nb 1-y Ta y O 3   (IC)
 
 wherein 
 A is Li, Na, K, Rb, Cs, Fr, or a mixture thereof; 
 B is Ba, Ca, Mg, Sr, or a mixture thereof; 
 0≦x≦0.3 and 0≦y<1, the values of x and y being independent of each other; 
 comprising spray pyrolising an aqueous solution, comprising alkali metal (A) ions, Nb ions and optionally alkaline earth ions (B) and/or Ta ions, so as to form a powder and calcining said powder. 
 
     
     
       4. A process according to  claim 1  wherein x=0. 
     
     
       5. A process according to  claim 1  wherein w=0. 
     
     
       6. A process according to  claim 1  wherein z=0. 
     
     
       7. A process according to  claim 1  wherein y=0. 
     
     
       8. A process according to  claim 1  wherein A is Na or K or a mixture thereof. 
     
     
       9. A process according to  claim 1  wherein y=0-0.5. 
     
     
       10. A process according to  claim 1  wherein spray pyrolysis is performed by atomisation of the solution into a furnace at a temperature of at least 800° C. 
     
     
       11. A process according to  claim 1  wherein the aqueous solution is prepared from water soluble precursors comprising at least one alkali metal nitrate or oxalate. 
     
     
       12. A process according to  claim 11  wherein said precursors are selected from the group consisting of sodium nitrate, potassium nitrate, ammonium niobium dioxalate ((NH4)NbO(C2O4)2) and tantalum oxalate. 
     
     
       13. A process according to  claim 1  in which said niobium compound forms a free flowing powder directly after spray pyrolysis. 
     
     
       14. A process according to claim in  13  which said niobium compound formed directly after spray pyrolysis does not adhere to any surface. 
     
     
       15. A process according to  claim 1  in which spray pyrolysis is carried out using a two phase nozzle arrangement. 
     
     
       16. A process according to  claim 1  in which the particles obtained directly after spray pyrolysis are perovskite or tungsten bronze in structure. 
     
     
       17. A process according to  claim 1  further comprising collecting the spray pyrolysis fine powder product by cyclone and calcining at a temperature in the range of 500-1000° C. 
     
     
       18. A process as claimed in  claim 17  wherein said calcination is carried out at a temperature of 550 to 850° C. 
     
     
       19. A process according  claim 1  further comprising forming calcined, spray pyrolysed particles into a green body and sintering said green body. 
     
     
       20. A process as claimed in  claim 1  further comprising milling of the powder after calcination. 
     
     
       21. A process as claimed in  claim 1  wherein Nb ions are provided via a Nb oxalate or dioxalate. 
     
     
       22. A process as claimed in  claim 1  wherein Nb ions are provided via NH 4 NbO(C 2 O 4 ) 2 . 
     
     
       23. A process as claimed in  claim 1  for the preparation of a compound of formula (IV)
   Na 1-x K x NbO 3   (IV)
 
 where x is between 0 and 1. 
 
     
     
       24. A process as claimed in  claim 1  for the preparation of a compound of formula (IV)
   Na 1-x K x NbO 3   (IV)
 
 where x is between 0.4 and 0.6.